Airborne radars typically employ RF phased array antennas. One type of phased array antenna is formed of an array of flared notch radiating elements operating at X-band. For military aircraft, it is desirable to have a low radar cross section (RCS) to reduce aircraft observability.
Aircraft also employ radio transponder equipment to perform Identify Friend or Foe (IFF) functions. Such transponder equipment typically operates at a lower frequency band, e.g., UHF or L-band, than the frequencies of operation of the radar systems.
It would represent an advantage to provide an array of low frequency radiating elements which share aperture area with a flared notch array radar antenna without compromising RCS or the active RF performance of the radar antenna.